To prevent global warming, the reduction of CO2 emission has recently been required, so that improvements in fuel efficiency and combustion efficiency in automobile engines have been pursued. Particularly, to improve fuel efficiency, improvements have been made to reduce friction in sliding members of pistons. The tension reduction of piston rings is important to achieve friction reduction. Because the tension of an oil control ring occupies 50% or more of the total tension of all piston rings, the reduction of its tension is significant. However, the reduction of the tension of an oil control ring leads to decrease in its followability to a cylinder wall surface, likely increasing oil consumption. Also, improvement in combustion efficiency by elevating the combustion temperature or by direct injection of a fuel into the combustion chamber of an engine leads to the degradation of a lubricating oil, so that the resultant oil sludge makes side rails stick to a spacer expander, wearing seating tabs of the spacer expander.
To reduce oil consumption, for example, JP 2003-286898 A proposes the reduction of the width of a pressure ring to 1.0 mm or less, and the width of a combined oil ring to 2.0 mm or less. Also, to prevent sticking with oil sludge, JP 2000-27995 A proposes the formation of a fluorine-containing, oil-repellent coating on a surface of the oil ring, the coating being formed by a sol-gel method from a metal alkoxide and fluoroalkyl-substituted metal alkoxide, in which part of alkoxyl groups are substituted by fluoroalkyl groups. Further, to suppress the wearing of seating tabs of a spacer expander, JP 56-66429 A proposes the soft-nitriding treatment of seating tabs, and WO 2005/040645 A proposes that a gas-nitrided layer containing an S phase having peaks at 2θ of 40° and 46° in X-ray diffraction with Cu—Kα is formed on seating tabs.
However, pursuing the reduction of tension inevitably encounters increase in oil consumption, making difficult the improvement of a combined oil ring without oil consumption increase.